We use EVOLVE-3D to simulate film profile evolution during an isolation trench refill process, focusing on the corner regions of the trench. EVOLVE-3D is a first principles, physically based simulator for low pressure deposition processes, i.e. the chemical reaction rate expressions used were developed from independent experiments. The trenches are refilled using two low pressure deposition processes : (1) low pressure chemical vapor deposition (LPCVD) of silicon dioxide from tetraethoxysilane (TEOS); (2) LPCVD of polysilicon (poly-Si) from silane. Three-dimensional deposition process simulations are used to estimate the film profiles after refilling in the idealized trenches with different aspect ratios. We find that the straight portions close before the trench corners close. If enough poly-Si is deposited to close the corners, the top of the void in the poly-Si moves up as a trench corner is approached. The position dependence of the void top should be considered in the optimization of the trench process, in order to avoid reopening trench corners when the poly-Si is etched back.